1. Field of the Invention
The present invention is related to battery charging circuits and, more particularly, to battery charging control circuits based on CMOS technology.
2. Related Art
Most portable electronics require batteries to supply power. When batteries are discharged to a low voltage level, chargers are required to charge the batteries to working condition. Li-Ion batteries need to be charged to about 4.2 V and NiMH/NiCd batteries need to be charged to about 5 V.
A battery charging control circuit controls the charging sequence to ensure that the charger safely charges the battery from a deeply discharged state to a fully charged state. There are at least two steps in the charging sequence, a slow charging mode and a fast charging mode. The battery charging control circuit initiates a charging mode according to the threshold voltage of the battery. For example, the threshold voltage of a Li-Ion battery is about 2.7 V. When the battery voltage is below the threshold voltage, the battery charging control circuit initiates the slow charging mode for safety. The slow charging mode current is about 40 mA. Because the voltage level is too low in this mode, the battery should not power external devices or the battery charging control circuit. The charger usually powers the battery charging control circuit in the slow charging mode. When the battery voltage is above the threshold voltage, the battery charging control circuit initiates the fast charging mode. The fast charging mode current is typically around 1 A. In this mode, the battery can power external devices and the battery charging control circuit.
A problem with this approach occurs if the battery charging control circuit is implemented with low-voltage CMOS technology. For example, the oxide breakdown voltage for 0.35 Φm CMOS technology is typically 3.3 V. In the slow charging mode, the charger is the only available power source to power the charging control circuit, but the voltage level of the charger can go as high as 13 V, which is substantially higher than the breakdown tolerance of low-voltage CMOS technology. One solution to this problem is to add an external voltage regulator to step down the charger voltage to within the breakdown tolerance of the low-voltage CMOS technology. Another solution is to implement the charging control circuit with special high-voltage CMOS or other technologies. But the problem with these solutions is increased cost and power consumption.
What is needed are methods and systems for controlling the charging of a battery that are compatible with low-voltage CMOS technology.